This invention relates to an apparatus for alignment of a beam of light with respect to a wafer on which high resolution patterns are to be defined and more particularly to an apparatus including a beam splitter for through-the-lens, visible light alignment.
In general, lithography refers to processes for pattern transfer between is various media. A lithographic coating is generally a radiation-sensitized coating suitable for receiving a projected image of the subject pattern. Once the image is projected, it is indelibly formed in the coating. The projected image may be either a negative or a positive of the subject pattern. Typically, a xe2x80x9ctransparencyxe2x80x9d of the subject pattern is made having areas which are selectively transparent, opaque, reflective, or non-reflective to the xe2x80x9cprojectingxe2x80x9d radiation. Exposure of the coating through the transparency causes the image area to become selectively crosslinked and consequently either more or less soluble (depending on the coating) in a particular solvent developer. The more soluble (i.e., uncrosslinked) areas are removed in the developing process to leave the pattern image in the coating as less soluble crosslinked polymer.
Projection lithography is a powerful and essential tool for microelectronics processing and has supplanted proximity printing. xe2x80x9cLongxe2x80x9d or xe2x80x9csoftxe2x80x9d x-rays (a.k.a. Extreme UV) (wavelength range of xcex=100 to 200 xc3x85) are now at the forefront of research in efforts to achieve the smaller desired feature sizes. With projection photolithography, a reticle (or mask) is imaged through a reduction-projection lens onto a wafer. Reticles for EUV projection lithography typically comprise a silicon substrate coated with an x-ray reflective material and an optical pattern fabricated from an x-ray absorbing material that is formed on the reflective material. In operation, EUV radiation from the condenser is projected toward the surface of the reticle and radiation is reflected from those areas of the reticle reflective surface which are exposed, i.e., not covered by the x-ray absorbing material. The reflected radiation effectively transcribes the pattern from the reticle to the wafer positioned downstream from the reticle.
It is well known that direct writing of a resist-coated semiconductor wafer with a beam of radiation can be employed to define high-resolution features for, for example, a very-large-scale-integrated circuit. In practice, circuit features are successively defined in the resist in a series of exposure steps. Following each exposure step, standard developing and processing steps such as etching, deposition, diffusion, etc., are carried out to form a prescribed pattern in the wafer. In this way, patterns are formed at successive so-called levels in the overall fabrication sequence. It is apparent that the patterns formed at these respective levels must be accurately aligned or registered with respect to each other.
In UV and DUV optical lithography, it is known to form alignment marks on the mask and the wafer and to employ the marks for precisely registering the mask with respect to the wafer. In this way, the two are accurately located in preparation for a subsequent patterning operation. During the registration step, the alignment marks are scanned by the beam in both the X and Y directions. Electrons backscattered from the scanned marks are detected and utilized to generate electrical signals. In turn, these signals serve as the basis for precisely positioning the beam with respect to the wafer.
In current lithography tools the indirect method of mask-to wafer alignment is most commonly used. This approach requires three steps. In the first step a stage alignment reference (SAR), that is located on the stage, is used to sense an alignment mark imaged from the mask by the lithographic projection system. This step locates the stage relative to the aerial image of the mask. In the second step an alignment microscope, that is attached to the projection system, senses the SAR. This step locates the stage relative to the alignment microscope. In the final step one or more alignment marks on the wafer are detected by the alignment microscope. This step locates the alignment microscope relative to the wafer. Taken together these three steps locate the alignment marks on the wafer to the aerial image of the mask as desired. This three-step process is time consuming and there are errors associated with each step. The inventive beam splitter enables a single-step through-the-lens alignment process.
Lithography systems are telecentric at the wafer, so viewing the superimposed alignment marks of the mask and wafer in sharp detail is problematic. The inventive beam splitter allows these images to be relayed to a viewing system without perturbing the alignment.
In one embodiment, the invention is directed to a beam splitter for transmitting and reflecting electromagnetic radiation from a source of electromagnetic radiation that includes:
a substrate having a first surface facing the source of electromagnetic radiation and second surface that is reflective of said electromagnetic radiation, wherein the substrate defines an anti-symmetric hole pattern about a central line on the substrate and symmetric in the perpendicular direction, and wherein the pattern comprises at least one hole through the substrate with the aperture of the hole being located at a defined first distance from the central point and the pattern further comprises an area, which is located at the same first distance but on the opposite side of the central point from the aperture, that defines a reflective solid surface.
In another embodiment, the invention is directed to a device for mask-to-wafer alignment that includes:
a source of electromagnetic radiation;
a substrate having a first surface facing the source of electromagnetic radiation and second surface that is reflective of said electromagnetic radiation, wherein the substrate defines a hole pattern about a central point of the substrate, and wherein the pattern comprises at least one hole through the substrate with the aperture of the hole being located at a defined first distance from the central point and the pattern further comprises an area, which is located at the same first distance but on the opposite side of the central point from the aperture, that defines a reflective solid surface;
means for projecting electromagnetic radiation containing a roughly centered image of a mask pattern toward the first surface of the substrate;
a wafer having a wafer pattern on its surface wherein the wafer is positioned downstream from the substrate so that electromagnetic radiation that passes through the at one least hole of the substrate is reflected from the wafer to produce reflected electromagnetic radiation containing superimposed images of the mask pattern and of the wafer pattern toward the reflective second surface of the substrate; and following this reflection,
means for detecting the superimposed images.
In a further embodiment, the invention is directed to a method for mask-to-wafer alignment that includes the steps of:
providing a mask that has a mask pattern on its surface;
providing a wafer that has a wafer pattern on its surface;
projecting electromagnetic radiation containing an image of the mask pattern through a beam splitter that comprises a substrate having a first surface and a reflective second surface whereby electromagnetic radiation that traverses through at least one hole that is formed through the substrate is reflected off the wafer surface to form a reflected electromagnetic radiation containing superimposed images of the mask pattern and of the wafer pattern that is directed toward the reflective second surface of the substrate; and
detecting the superimposed images.